Download The Criterion for the Tangential Sensitivity Measurement Application

The Criterion for the
Tangential Sensitivity Measurement
Application Note 956-1
A tangential signal is defined on a C.R.T. display as a pulse whose
bottom level coincides with the top level of the noise on either side of
the pulse (Figure 1). Although the corresponding signal-to-noise ratio
depends on many system factors, the generally accepted ratio of 8 dB
at the output correlates well with the tangential appearance on the
oscilloscope for practical systems.
PULSED SIGNAL AT
RECEIVER OUTPUT
NOISE (SOMEWHAT IDEALIZED)
TANGENTIAL SIGNAL
Figure 1.
The often asked question concerning whether 8 dB refers to voltage or
power is not a valid one. The number of decibels is defined as 10 log10
(P1 ÷ P2) where P1 and P2 are power levels to be compared. If output
voltages are to be compared, the ratio (V1 ÷ V2)2 may be substituted for
(P1 ÷ P2). In this case the number of decibels is 20 log10 (V1 ÷ V2).
The number of decibels determines both (V1 ÷ V2) and (P1 ÷ P2). The
terms “voltage dB” and “power dB” are not significant. For example,
the 8 dB output ratio corresponds to a power ratio of 6.3 and a voltage
ratio of 2.5.
Another source of confusion is the relationship between input ratios
and output ratios. Because the detector is a square law device, the
output voltage is proportional to the square of the input voltage, or to
the input power. A signal-to-noise voltage ratio of 2.5 at the output thus
corresponds to an input power ratio of 2.5. Since 10 log 2.5 = 4, the
equivalent input signal-to-noise ratio for tangential sensitivity is 4 dB.
SIGNAL
GENERATOR
DUT
VIDEO AMPLIFIER
RMS VOLTMETER
HP 3400A
BIAS SUPPLY
Figure 2. TSS Test System
A useful production test system (Figure 2) uses an RMS voltmeter to
compare signal output to noise output. The noise level is observed on
the meter with R.F. signal off, but with d.c. bias applied to the D.U.T.
Then the specified tangential signal level is applied and the increase in
RMS voltmeter reading must correspond to 8 dB or more.
The use of square wave modulation and AC coupling introduces
another source of confusion to this measurement. The increase in
reading on the RMS voltmeter corresponding to the 8 dB criterion is
4.1 dB. The 8 dB criterion means that the peak signal voltage is 2.5
times the RMS noise voltage Vm. Because the RMS meter uses AC
coupling, the square wave is symmetrical with amplitude 1.25 Vm. The
square of this voltage combines with the square of the noise voltage to
give the total voltage on the RMS meter.
VT2 = VN2 + (1.25 VN)2 = 2.56 VN2
This ratio corresponds to 4.1 dB.
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(408) 654-8675
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Data Subject to Change
Copyright © 1990 Hewlett-Packard Co.
Obsoletes 5952-0423
Printed in U.S.A. 5091-0169E (10/90)